1. Field of Invention
The present invention relates generally to an underwater breathing device and, in particular, to an exhalation valve for use in an underwater breathing device that is configured to produce positive end-expiratory pressure in the airway of a user.
2. Description of Related Art
An underwater breathing device enables a user to continue breathing even after the user's mouth and/or nose is submerged in water. Some underwater breathing devices, such as scuba and snuba breathing devices, are configured to provide a submerged user with air from a compressed-air source. Other underwater breathing devices, such as a conventional snorkel, are configured to provide a user with air from the atmosphere.
A conventional snorkel generally includes a breathing tube through which air can be inhaled from the atmosphere. The breathing tube is typically configured with two ends. One end of the snorkel is intended to remain above the surface of the water. The other end of the snorkel is intended to be submerged under the surface of the water. The end of the breathing tube that is intended to be submerged generally includes a mouthpiece. In practice the user inserts a portion of the mouthpiece into his mouth and thereby creates a seal between the user's airway and the breathing tube. The user then submerges his mouth and the mouthpiece under water while maintaining the other end of the breathing tube above the surface of the water, thereby enabling the user to inhale atmospheric air while submerged in water. At the same time, the breathing tube enables the user to exhale through the user's mouth without breaking the seal between the user's mouth and the mouthpiece. Generally, the air exhaled by a user exits the snorkel through the same breathing tube through which the user inhales atmospheric air.
One problem that a user can encounter while using a conventional snorkel is increased fatigue due to the compressive forces of the ambient water in which the user is submerged. During normal inhalation and exhalation, a user expends effort inflating and deflating his lungs. When a user is submerged in water, however, the compressive forces of the ambient water around the user's chest force the user to expend more effort than usual in order to inflate his lungs and tend to cause the user to expend less effort than usual to deflate his lungs. This reduced-effort exhalation tends to cause the user to exhale faster than normal and down to smaller residual lung volumes than normal such that there is less time between each inhalation, resulting in more frequent inhalation. More frequent inhalation can cause the user's inhalation muscles to fatigue relative to normal inhalation and exhalation, which can result in a smaller functional lung capacity, the possibility of atelectasis, and increased breathing difficulty.
Another problem that a user can encounter while using a conventional snorkel is difficulty breathing due to water being present in the breathing tube of the snorkel. Water can sometimes enter a conventional snorkel through one or both ends of the breathing tube. This water can cause difficulty breathing when it accumulates to the point where the water interferes with the passage of air in the breathing tube and/or the water is inhaled by the user. In addition, the presence of water in the breathing tube of the snorkel can cause a distracting gurgling or bubbling noise as air passes by the water during inhalation and/or exhalation.